Modification procedures wherein SiOR group containing polysiloxane resin precursors of the indicated kind are reacted with organic COH group containing compounds, are well known and have been described in the chemical literature. Reference is thus had to "Chemie und Technologie der Silicone" by Walter Noll, page 215 ff. and page 262 ff. The purpose of the modification is to produce resin compositions which combine the beneficial characteristics of the initial polysiloxane with those of the organic resins. The siloxane component in the ultimate resin composition causes primarily a marked improvement of the thermal stability and of the weather resistance. Thus, the resistance towards yellowing or other discolorisation and the retention of gloss when subjected to thermal stresses are markedly improved by the modification. The organic resin component, in turn, imparts the modified resin primarily with improved adhesion or bonding characteristics, improved durability and also satisfactory formability. Moreover, the presence of the organic resin component contributes to satisfactory pigment compatibility.
According to the prior art, numerous organic COH group containing resin precursors have been proposed. Particularly suitable are polyesters as they are obtained by reaction of adipic acid, terephthalic acid, isophthalic acid, phthalic acid or their esters -- for example, methyl esters -- or anhydrides with multivalent alcohols, such as, for example, ethyleneglycol, trimethylolpropane, glycerine, and pentaerythrite. Suitable for the intended purpose are also acrylic resins, epoxy resins, polycarbonates, phenol resins, melamine resins or urea resins with free COH groups. Of particular importance are alkyde resins which contain esters of carboxylic acids containing several unsaturated bonds.
The cocondensation between the organosilicon compound and the organic resin proceeds pursuant to the following reaction scheme: EQU .tbd.SiOR.sup.1 + .tbd.COH .fwdarw..tbd.SiOC .tbd. + R.sup.1 OH
this reaction can be catalyzed by suitable catalysts as, for example, by zirconium acid ester and titanium acid ester. Trifluoro acetic acid is also a suitable catalyst. The reaction may be carried out in the presence of inert solvent. Suitable examples for such solvents are cyclohexanon, xylene and toluene.
The cocondensation is suitably carried out at temperatures above 70.degree. C., preferably in a temperature range between 120.degree. C. and 150.degree. C.